Hydrocarbon feedstocks intended for use in synthesis gas generation must first be desulphurised in order to protect sensitive downstream catalysts from deactivation. Sulphur removal is conventionally carried out using a combination of hydrodesulphurisation (HDS) catalysis (based typically on CoMo or NiMo catalysts) followed by zinc oxide based absorbents. In this process, the HDS catalyst converts organosulphur species such as mercaptans and alkyl-sulphides in the feed to hydrogen sulphide, which may be subsequently removed, along with existent hydrogen sulphide, by reaction with the ZnO. In order for the hydrodesulphurisation reaction to occur, it is necessary to have hydrogen present in the feed, and this is often provided by recycling a portion of the synthesis gas or hydrogen generated downstream. Although this two-stage process can be effective, typically providing protection down to <100 ppbv sulphur, some sulphur slip will invariably still occur resulting in gradual deactivation of downstream catalysts. This is particularly problematic for pre-reforming and noble metal based steam-reforming catalysts, as these are highly susceptible to sulphur. Furthermore some sulphur compounds such as thiophene are resistant to hydrogenation over Co and Ni catalysts.
EP-A-0320979 discloses a process for steam reforming of hydrocarbons after desulfurizing the hydrocarbon to a sulfur content of 5 ppb or less using a three-stage process in which a mixture of hydrocarbon and hydrogen is firstly subjected to hydrodesulphurisation over a Ni—Mo or Co—Mo catalyst at 350-400° C., and secondly hydrogen sulphide absorption over a zinc oxide absorbent at a temperature in the range 250-400° C. Finally, the resulting mixture is passed over a copper-zinc desulphurisation material formed by co-precipitating copper and zinc salts, optionally also in the presence of an aluminium salt, with an alkaline material, drying and calcining the precipitate, and then reducing the calcined material with a hydrogen/nitrogen gas mixture.
EP-A-1192981 discloses a desulphurisation material comprising copper, zinc oxide, alumina and iron or nickel compounds, formed by co-precipitating copper and zinc compounds with an alkali substance, drying and calcining the precipitate, shaping the oxidic material, impregnating the shaped material with iron and/or nickel compounds, calcining the impregnated material to convert the iron or nickel compounds to their oxides and then reducing the material with hydrogen. The desulphurisation materials disclosed contained in the region of 40% wt copper and 1 to 10% wt iron and/or nickel.
The desulphurisation material preparation procedures described above are complex and the latter requires two calcination steps, which add considerably to the manufacturing cost.
In practice, the copper-zinc desulphurisation materials are generally provided as the calcined oxidic materials, which are reduced to their active form in-situ. The high copper content in the desulphurisation materials, which is typically over 30% by weight expressed as copper oxide, leads to lengthy and cumbersome procedures due to the highly exothermic nature of the copper oxide reduction. Furthermore, although these products are able to remove residual sulphur species for a limited period, their effectiveness diminishes significantly with time on line. They also suffer from low ultimate sulphur capacity and a maximum of about 1 wt % S is typical. Moreover there is a desire to reduce the metal content in such materials while retaining their effectiveness and also to simplify the production route to reduce cost. However the products need to maintain their effectiveness and possess suitable physical properties such as crush strength in order to be effective replacements.